Heathkit AA-151 project. Help!

Discussion in 'Tube Audio' started by hugh811, Jun 14, 2018.

  1. hugh811

    hugh811 New Member

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    Hi,

    I have been a while on the project to restore a Heathkit AA-151. With much fun!

    Some replacements already done: replaced old capacitors and resistors that are out of tolerance range; new el84 power output tube set; Deoxit used for pots.

    Other preamp tubes seem to be good according to my tube tester.

    It is much better now. It sounds good, but there is still a "slight" 60hz constant hum out of both speakers. Hum is even out of both speakers. I described it as "slight" as the amp is hooked up to a high sensitivity 16 Ohm JBL D130. The hum is "independent", constant at all volume levels.

    The old coupling capacitors still there, because they are still within tolerance range. I think that the problem lies in these coupling capacitors or big power filter capacitors.

    Please help me to identify the main suspect and what to try first. It could save time and labor hours.

    I thank you. Appreciate your help!
     
    Last edited: Jun 14, 2018
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  2. s-petersen

    s-petersen Scott Subscriber

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    Hum in both channels is usually the big filter cans, The coupling caps should be checked for leakage at their rated voltage, or just simply replaced, as they are a low cost item.
     
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  3. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Two things to check -- that the output tubes are balanced, and that the 6AU6 tubes don't have any heater to cathode leakage. Both will cause hum in that unit.

    Dave
     
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  4. hugh811

    hugh811 New Member

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    Thanks for advice. I’ve just finished the project after a while. I replaced the two 6au6 and the hum has gone. Nice amp and it looks very vintage.
     
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  5. Rational35

    Rational35 AK Subscriber Subscriber

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    I also have an AA-151 which I picked up in a barter deal, oh about 20 years back. Nice little amp.

    But it developed that same constant hum symptom too, after 10 years of trouble-free service. Replacing the filter caps & 5AR4 did nothing, and the EL84s tested pretty close, so I suspected h-k leakage in one of the preamp tubes.

    I never got around to running all the tubes thru my AVO CT160 to find the culprit, but this just gave me some incentive to cast a wary eye at the 6AU6s. Time to replace all the original coupling caps, too.
    Thanks for that hint, Dave!

    Way back, I replaced the single cathode resistor (100 ohm, I think) with a 220 ohm per channel pair, with 470uF bypasses. (this is all from memory) That way, all I have to worry about is matched pairs of EL84s for each channel, not necessarily a matched quartet. But a matched quad of 7189s is in the near future, I think.

    The AA-151 is a sweet-sounding amp. By all accounts, those 51-29 output trannies were some of the finest ever wound for EL84s.

    Peter
     
    Last edited: Oct 30, 2018
  6. hugh811

    hugh811 New Member

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    @Rational35 : you should check all the four 470k resistors that are connected to El84 tubes. Best scenario is that they should still be in their range of tolerance and most important that they are nearly matched on your capacitance checker. It will absolutely work. I had changed two out of the four. now they are “matched” capacitors and have almost the same reading on my vol meter. Hugh

    @ Multisection Filter Capacitors change is only the last resort for me. It is recommended to have them checked. But in most cases, i think they are still acceptable for me.
     

     

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  7. triode17

    triode17 Super Member

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    Wait! You started talking about 470k resistors and ended with matched capacitors ??? Huh? please explain.
     
  8. hugh811

    hugh811 New Member

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    Sorry. My mistake. I mean: “matched resistors”, not capacitors. Not capacitance checker. Voltmeter. Just try to explain what i have done with my AA-151. I am just an amateur trying to spend spare time with vintage audio stuff. The AA-151 found its way to my home in an imperfect technical state. The sound was blurred, one channel was heavily distorted, hum noise came out from both channels. Now it is much better, sweet as it should be, unnoticed noise on a highly sensitive pair of speakers. Crazy thing is that it still has its original power filter and coupling capacitors. They are still within their range of tolerance. But quite a number of resistors have been replaced. even if only one resistor had a problem, i changed the whole pair on both channels.
     
  9. hugh811

    hugh811 New Member

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    Just a few old capacitors have been replaced, two or three of them.

    But it doesn’t mean that there is no need to check capacitors. They all should be checked through.
     
  10. s-petersen

    s-petersen Scott Subscriber

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    If the coupling caps are paper, they should be changed, because they likely leak at operating voltage and change the bias to the tubes.
     
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  11. hugh811

    hugh811 New Member

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    You are absolutely right. I had replaced many paper capacitors in heatkit, scott, eico tube amp. just want to say that Before taking the decision, i checked capacitors and the voltage stated in the schematics. If it is ok, then i leave as it is.

    If it fails, so no other choice than to replace them all. But as i mentioned, it is only the last resort for me. A scott 299 or fisher 500C without old german/usa -made capacitors ? I don’t like it. I think High quality old capacitors are one of the reasons to make these more than 50 year old amplifiers still alive and sought after.

    With new capacitors, it would sound with more details like a modern stuff. Vintage gears = should take acceptable risks. Modern gears = no worry about failure.

    But I still love vintage tube gears.
     

     

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  12. Big Harry

    Big Harry Active Member

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    I own 2 AA151's and have replaced most of the caps and resistors in both of them because the original parts were either suspect or down right out of tolerance. I would rather have an amp that is more or less reliable as one of them is in my garage and can be left on for several hours at a time and I don't trust parts that have been around for 55+ years to be reliable enough to be in that kind of service. Caps and resistors are a whole bunch cheaper that trying to find a replacement power or output transformer because a cap failed and took out an expensive hard to find part.
     
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  13. Rational35

    Rational35 AK Subscriber Subscriber

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    When I first got my AA151, I replaced all the high-value plate resistors in the pre-amp stages (and quite a few others) as a matter of course. But I confess, I don't think I checked the EL84 grid resistors. I'll take a look. The multi-section electrolytics tested good for capacitance, leakage, and ESR. While troubleshooting, I bypassed them with modern Nichicons. No change.

    I still suspect h-k leakage in a 6au6 is the hum culprit. It's 60Hz hum, not 120Hz ripple. Just bought a bunch of 6au6s off that auction site, so I have a few to roll for noise. If I actually find some spare time, I'll run em through my AVO CT160 tube tester ;) That's just such a satisfying, but time-consuming ritual.

    I also have a pending shopping cart at Mouser, to replace every single Daystrom paper coupling cap with new Illinois Capacitor polyprop axial tubulars. It's insurance.

    One of the next projects will be refurbishing a cosmetically-perfect matching AJ-11 tuner which arrived at my doorstep DOA. Not that I listen to the radio much anymore, except on the road. But seeing/hearing that amp/tuner pair stacked and running a pair of Altec A7s would make me feel young again :)

    Peter
     
    Last edited: Nov 26, 2018
  14. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    The 6AU6's are always a suspect for 60 Hz hum. If you think yours are the problem, operate the amplifier with these tubes removed and see if the hum goes away. If so, you've found your problem!

    Dave
     
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